Polyamide as a Denture Base Material: A Literature Review.

The purpose of this article was to review the biocompatibility, physical, and mechanical properties of the polyamide denture base materials. An electronic search of scientific papers from 1990-2014 was carried out using PubMed, Scopus and Wiley Inter Science engines using the search terms "nylon denture base" and "polyamide denture base". Searching the key words yielded a total of 82 articles. By application of inclusion criteria, the obtained results were further reduced to 24 citations recruited in this review. Several studies have evaluated various properties of polyamide (nylon) denture base materials. According to the results of the studies, currently, thermo-injectable, high impact, flexible or semi-flexible polyamide is thought to be an alternative to the conventional acrylic resins due to its esthetic and functional characteristics and physicochemical qualities. It would be justifiable to use this material for denture fabrication in some cases such as severe soft/ hard tissue undercuts, unexplained repeated fracture of denture, in aesthetic-concerned patients, those who have allergy to other denture base materials, and in patients with microstomia.  Although polyamide has some attractive advantages, they require modifications to produce consistently better properties than the current polymethyl methacrylate (PMMA) materials. Moreover, since there is a very limited knowledge about their clinical performance, strict and careful follow-up evaluation of the patients rehabilitated with polyamide prosthesis is recommended.


Introduction
Polymethyl methacrylate (PMMA) has been the most popular material used for denture fabrication since its introduction in 1937. [1] It has several advantages such as an excellent esthetic characteristic, low water sorption and solubility, adequate strength, low toxicity, easy repair, and a simple molding processing technique.
Nonetheless, it has some problems such as polymerization shrinkage, weak flexural, lower impact strength, and low fatigue resistance. [1][2][3][4] These often lead to denture failure during chewing or when fall out of the patient's hand. In order to enhance some properties of PMMA, various efforts have been taken including addi-tion of metal wires or plates, fibers, [5][6][7][8] metal inserts, [9] and modification of chemical structure. In recent years, nylon polymer has attracted attention as a denture base material. Polyamide resin was proposed as a denture base material in the 1950s. [10] Nylon is a generic name for certain types of thermoplastic polymers belonging to the class known as polyamides. These polyamides are produced by the condensation reactions between a diamine NH 2 -(CH 2 ) 6 -NH 2 and a dibasic acid, CO2H-(CH 2 ) 4 -COOH. [11][12][13][14][15] Nylon is a crystalline polymer, whereas PMMA is amorphous. This crystalline effect accounts for the lack of solubility of nylon in solvents, as well as high heat resistance and high strength coupled with ductility. [16][17] Moreover, it was claimed that nylon materials have other advantages including higher elasticity than common heatpolymerizing resins, [12] toxicological safety for patients with resin monomer and metal allergy, [18] use of heat-molding instead of chemical polymerization to control the polymerization shrinkage and its related deformation. [18] On the other side, it is reported that this material has several problems such as water sorption, surface roughness, bacterial contamination, warpage, color deterioration, and difficulty in polishing. [19] The present study is a literature review to appraise some physical, mechanical and clinical properties of nylon/ polyamide denture base materials.

Method
This study is a structured literature review of articles published from 1990 to 2014. PubMed, Scopus, and Wiley Inter Science databases were used to search "nylon denture base" and "polyamide denture base" key words. The search was limited to English language publications. The articles were reviewed by two experts in the field of prosthodontics. Searching the key words yielded a total of 82 articles. As the inclusion criteria, the publications had to be exactly related to the key words; no editorials and manufacturer-supported publications were accepted for review process. By application of inclusion criteria, the obtained results further reduced to 24 citations that formed the basis for this review ( Figure 1). strength. This study demonstrated that the mechanical properties of injection-molded thermoplastic denture bases differ from each other; hence, the clinicians should be well aware of these properties in order to choose the most suitable one for an RPD without metal clasps that is suitable for each patient.
In 2012, Ucar et al. [22] evaluated some mechanical properties of a polyamide-based denture material (Deflex) and contrast it with another injection-molded PMMA base material and a conventional compressionmolded PMMA. The results revealed that the polyamide flexural strength was not significantly different from compression-molded PMMA and that the flexural modulus of polyamide was lower than compression-molded PMMA material. The major connector of a removable denture should be rigid enough to evenly distribute the applied force on the denture. Therefore, it was emphasized that a lower flexural modulus (higher flexibility) has been often a disadvantage from clinical standpoint. [23][24][25] Another study in 2012 [26] reported that ther- In 2013, Wadachi et al. [28] compared the rigidity of dentures made of a polyamide resin (Valplast), a polyester resin (Esthe Shot) and a conventional heatpolymerized resin (Physio Resin). They showed that the denture made of polyamide resin had the lowest elasticity; therefore, it was reasonable to think that this material allowed for the most considerable displacement of denture and permitted the pressure to be applied under the denture base. They concluded that this material needed to be reinforced by using metal frames in order to prevent the deformations caused by occlusal forces.

Water sorption and water solubility
Lai et al. in 2003, [29] studied the color stability, stain resistance and water sorption of one silicon, one copolyamide, and two heat-cured acrylic resins as removable gingival flange materials. In that study, copolyamide (Flexite Supreme) had absorbed the greatest amount of water, whereas silicone showed the least water uptake after 56 days of water storage.
In the study carried out by Takabayashi in 2010, [20] water sorption of two of the tested polyamide materials (Valplast and Flexite Supreme) met the ISO standard (32 µg/mm 3 ), but Lucitone FRS revealed the highest water sorption due to the greater degree of hydrophilic characteristics supported by the contact angle measurements. It is thought that the higher the amide group concentration, the greater the water sorption. Therefore, it has been suggested that the amide group concentration, in the polyamide type denture base materials, could be adjusted to a level as low as that in popular industrial materials such as nylon 6 or 66. [20] On the other hand, in another study which was done by Shah et al. in 2014, the sorption and solubility of heat-cured polymethyl methacrylate denture base resin and flexible denture base resin were compared and it was found that heatcured PMMA had more sorption and solubility values than flexible (thermoplastic polyamide nylon) resin. [30] The study suggested that since the contact angle between the flexible resin and water was high with low surface free energy, their water repellency was also high, and these all resulted in lower water sorption values. Likewise, it was mentioned that there was a strong hydrogen bonding between amide groups and a reduction in attachment areas for water molecules; therefore, the amount of water sorption in flexible resin was lower than conventional PMMA. The higher residual monomer contents were mentioned as a cause for the higher solubility levels of PMMA. Cola, red wine, and distilled water) by using an ultraviolet-visible spectrophotometer. The most severe staining was shown with red wine followed by coffee; Transflex showed a significant color change after 15 and 30 days of immersion in Cola. The larger color changes for nylon denture base materials would be related to their hygroscopic and also higher water sorption properties. [32][33] It was found that the frequency of amide groups along the chain had affected the water sorption and the chemical properties of each type of nylon. [29] Another attributed reason could be the differences in finishing and polishing of nylon materials compared to PMMA.
Rougher surfaces are more susceptible to staining. [34][35][36] Bond strength to other materials Auto-polymerizing resin is often used as reline or repair material for PMMA denture base, [37]  available. [43] The study of Stafford et al. [18] was the first attempt to study the dimensional accuracy of nylon denture-base materials. Parvizi et al. in 2004 [44] compared the dimensional accuracy of an injection-molded nylon denture base material with one conventionally processed PMMA, one injection-molded PMMA, and an injection-molded styrene. They found that all of the materials exhibited some degree of shrinkage as a result of processing, but this shrinkage was highest for nylon with 2.5% in the cross arch dimension, which was 2.8 times greater than the conventionally processed PMMA.
The smallest mean shrinkage was associated with styrene and the largest with nylon. The dimensional change of nylon was clinically significant, and could have an effect on the final fit of the denture. The lower dimensional accuracy of nylon was shown to be related to its technique sensitivity during the processing stages. Also its dimensional change could be affected by water sorption when considering nylon as a hydrophilic material. [44][45] Surface roughness A rougher surface can cause discomfort to patients and also discoloration of the prosthesis; it may contribute to microbial colonization and biofilm formation. Pro-Cast DSP) denture bases by using scratch test. The results showed that the surface of thermoplastic denture base resins was easily damaged compared with polymethyl methacrylate.

Effect of denture cleansers on polyamide denture base materials
Adhesion of microorganisms, especially yeasts, to the denture base materials is an important issue that compromises its service and efficacy. [49] Although too many researches have been conducted to control the development of pathogenic biofilm on PMMA materials; [50][51][52]

Case reports
In 2013, [62] a clinical report described a combination of a nylon partial removable prosthesis and a traditional partial removable dental prosthesis for a Kennedy class II, modification 1, partially edentulous mandible. After two years, this combination was functioning well, although the nylon material surface showed some discoloration.
Sinch et al. in 2013 [63] reported a case of maxillary and mandibular arch reconstruction with a nylon denture base material due to the aesthetic concerns of the patient. They concluded that flexible partial dentures (FPD) could be a good option for replacement of missing teeth when the patient is concerned about aesthetics.
FPD had given an option of thinking beyond complex designing of cast partial dentures. They could be considered for treating any patients who need replacement of missing teeth with aesthetic concerns; however, proper care of the prosthesis must be taken in order to minimize the staining of the prosthesis, which otherwise affects the aesthetics of the prosthesis.

Conclusion
Physical and clinical properties of polyamides were briefly discussed in this review article. Although the flexural strength and modulus of elasticity and rigidity of nylon (polyamide) denture base materials are relatively low, they demonstrate great impact strength, toughness, and resistance to fracture. It was suggested that by adding glass fibers to polyamides, their stiffness and other mechanical properties could be increased. The use of these materials for non-metal clasp dentures has some advantages regarding their esthetic and degree of retention. However, these materials show some degree of color instability in different beverages. The bond strength of these materials to the repairing resins is low, but it can be significantly enhanced by silica coating with Rocatec system. Using the denture cleansers would increase the surface roughness of these materials and their cytotoxicity increases after long-term use. It was demonstrated that polyamides have rougher surface than other resin materials, and it causes more bacterial and fungal colonization. Therefore, selection of thermoplastic resin and designs adaptation for each clinical case must be achieved only after complete indulgence of the properties of polyamide materials.